1. Field of the Invention
The present invention relates to a driving device, a fixing device and an image forming apparatus.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor drum (image bearing member) with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor drum on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, and then heat and pressure are applied to the sheet at a fixing nip to form a toner image on the sheet.
Incidentally, for example, speedup of image forming apparatuses for production print markets has been increasingly advanced, and in particular, high output of the driving source (for example, a direct current (DC) motor) for driving the fixing roller in the fixing section is desired. Generally, such a driving source is specifically designed in accordance with the required output, and tends to be a dedicated product. This has resulted in imbalance between the design cost including the metal mold for the driving source and the production volume appropriate to the cost, and therefore reduction of production cost is an urgent necessity.
In addition, a stepping motor for highly accurate positioning is used as the driving source used for sheet conveyance. However, when the output of the stepping motor is increased in the case where the output is further increased to achieve printing of thick sheets and high-speed sheet conveyance, the inertia of the stepping motor is increased, and this makes it difficult to achieve highly accurate positioning. When a DC motor is used for the driving source to achieve high output, the positioning accuracy comparable to that of the stepping motor has to be ensured, and in view of this, there is room for improvement in achieving high output of the driving source.
To achieve high output of the driving source, Japanese Patent Application Laid-Open No. 2010-41747 discloses a driving device which achieves high output by use of a plurality of driving sources, for example. FIG. 1 illustrates a gear mechanism in the driving device using two driving sources M1 and M2.
For example, as illustrated in FIG. 1, in the case of the configuration for achieving high output by use of two driving sources M1 and M2, first driving gear G1 and second driving gear G2 connected with two shafts X1 and X2 of driving sources M1 and M2 engage with idle gear G3, and the driving force is transmitted to idle gear G3 from two driving sources M1 and M2. In this manner, idle gear G3 can output the driving force of the sum of the driving forces of two driving sources M1 and M2.
In some situation, a gear eccentrically moves along an ellipse path, not a perfect circle. The eccentricity of a gear is caused by, for example, a positional shift between the drive shaft of the driving source and the rotation center of the driving source, inclination of the drive shaft of the driving source, shift between the fixation position of a shaft that pivotally supports the gear and the rotation center of the gear, inclination of the shaft that pivotally supports the gear, shift between the position of the center of the shaft hole of the gear and the rotation center of the gear, inclination of the shaft hole of the gear, and the like.
In the configuration disclosed in Japanese Patent Application Laid-Open No. 2010-41747, in the case where the gears have no eccentricity, the engagement positions of the gears are not varied and the torques generated at the engagement portions of the driving gears and the idle gear are constant. In contrast, in the case where at least one of the gears has eccentricity, the engagement positions of the gears and the idle gear are varied and the torques generated at the engagement portions of the driving gears and the idle gear are not constant.
For example, in the configuration of FIG. 1, in the case where only second driving gear G2 has eccentricity, the engagement position of second driving gear G2 and idle gear G3 is vertically varied under the influence of the eccentricity of second driving gear G2. When the position is varied, torque B generated at the engagement portion of second driving gear G2 and idle gear G3 is varied depending on the position of second driving gear G2.
In addition, in the configuration of FIG. 1 where a plurality of driving gears are engaged with the idle gear, to keep the rotational speed constant, the driving sources operate to keep the total torque generated at the engagement portions of the gears constant. Therefore, when torque B is varied, torque A generated at the engagement portion of first driving gear G1 and idle gear G3 is also varied. FIG. 1 illustrates an example case where second driving gear G2 is located at a position where torque B is greater than torque A.
As described, in the configuration disclosed in Japanese Patent Application Laid-Open No. 2010-41747, the eccentricity sensitivity of the gears is high, and, when the gears have eccentricity, the balance of the torques at the engagement portions of the gears is poor, and consequently, the output of the idle gear may possibly be varied.
To be more specific, for example, in the case where the two driving sources are of the same type (for example, DC motors) in the configuration of FIG. 1, the output of idle gear G3 may become insufficient depending on the position of the gear having eccentricity. In addition, in the case where one of the driving sources is composed of a stepping motor and the other is composed of a DC motor, an excessive load is applied to the stepping motor side depending on the position of the gear having eccentricity, and in turn, loss of synchronization of the stepping motor may possibly result.